“Background Art 1”
With the rapid spread of portable appliances in recent years, small high-performance storage batteries have come to be strongly desired. In alkaline storage batteries represented by nickel/metal-hydride storage batteries, nickel-cadmium storage batteries, nickel-zinc storage batteries, and the like, a further increase in capacity and a further reduction in size are expected.
Incidentally, many techniques for improving a positive active material so as to attain a capacity increase in alkaline storage batteries have been proposed, for example, as shown below.
(1) In JP-B-7-77129, the pore volume of positive active material particles comprising nickel hydroxide is reduced to thereby heighten the density of the positive active material particles and thus attain an increased battery capacity.
(2) In JP-A-61-138458, a cobalt compound generating a bivalent cobalt ion in an alkaline electrolytic solution is added to and mixed with positive active material particles comprising nickel hydroxide. According to this technique, the cobalt compound first dissolves in an alkaline electrolytic solution and thereafter reprecipitates as cobalt hydroxide, upon which reprecipitation the cobalt compound forms a network connecting the positive active material particles and the current collector to one another. This network changes into a satisfactory conductive network when the cobalt hydroxide is oxidized into a highly conductive cobalt compound in the first charge of the battery. Consequently, the coefficient of use of the active material is improved.
(3) In JP-A-63-152866 and JP-B-4-4698, positive active material particles comprising nickel hydroxide are coated with cobalt hydroxide beforehand. According to this technique, a satisfactory conductive network is formed by the first charge of the battery to attain an improved coefficient of use of the active material, as in the technique (2) described above. In addition, a cost reduction due to the reduced cobalt amount and an increase in energy density can be attained.
However, since the oxidation of the cobalt hydroxide by the first charge is an irreversible reaction, the cobalt hydroxide oxidized is not reduced during discharge. Because of this, the negative electrode has electricity remaining undischarged. The quantity of this electricity is referred to as “discharge reserve”. In nickel/metal-hydride storage batteries, a discharge reserve generates also upon corrosion of the negative-electrode alloy. Furthermore, a discharge reserve generates depending on the quantity of irreversible electricity in the oxidation/reduction of the nickel hydroxide.
When a discharge reserve generates, the uncharged capacity of the negative electrode at the final stage of charge (referred to as “charge reserve”) is reduced and this accelerates hydrogen gas evolution at the negative electrode to elevate the internal pressure of the battery. When the internal pressure increases, the safety vent works to release the gas. Because of this, the water constituting the electrolytic solution disappears and the battery comes to be deficient in electrolytic solution and have reduced performance. The battery life is thus shortened.
Furthermore, in nickel/metal-hydride storage batteries, the negative electrode is made to have a discharge reserve and a charge reserve by regulating the capacity of the negative electrode so as to be larger than that of the positive electrode. The discharge capacity of the battery is hence usually controlled by the positive electrode. Because of this, when an increase in battery capacity is attempted by increasing the capacity of the positive electrode, the capacity of the negative electrode also should be increased, making it impossible to attain a size reduction. If the generation of a discharge reserve can be inhibited, the amount of the negative active material to be used for impregnation can hence be reduced accordingly to attain a size reduction or the amount of the positive active material to be used for impregnation can be increased to attain a capacity increase.
Consequently, techniques for reducing a discharge reserve have been proposed in order to attain a higher performance by inhibiting the internal-pressure increase during charge or improving cycle life characteristics and to attain a size reduction and a capacity increase. For example,
(4) in JP-A-3-78965, JP-A-8-148145, and JP-A-8-213010, a high-order cobalt compound in which the cobalt has an oxidation number larger than 2 is disposed beforehand on the surface of positive active material particles comprising nickel hydroxide. According to this technique, the irreversible reaction in which cobalt hydroxide is oxidized by first charge is inhibited and, hence, a reduced discharge reserve is attained. However, this technique is insufficient because the irreversible capacity of cobalt hydroxide is not the only cause of discharge reserve generation.
Furthermore, the following has been proposed as a technique for attaining a reduced discharge reserve. Namely,
(5) in JP-A-2000-223119, an attempt has been made in which a high-order cobalt compound is deposited on the surface of positive active material particles comprising nickel hydroxide and an oxidation treatment with an oxidizing agent is conducted in an aqueous alkali solution to partly oxidize the nickel hydroxide.
However, when the oxidation treatment disclosed in the patent document cited in (5) above is conducted in a high-concentration aqueous alkali solution at a high temperature, part of the nickel hydroxide is oxidized to γ-NiOOH and the tap density of the positive active material particles decreases. There is hence a problem that the decrease in tap density is contrary to the desired density increase in positive active material particles and this in turn is contrary to the desired capacity increase (first problem).
It was found that a technique effective in eliminating the first problem is to first conduct an oxidation treatment in an aqueous alkali solution of 20% by weight or lower containing an oxidizing agent under the conditions of 60° C. or lower, thereafter add an aqueous alkali solution of 30% by weight or higher, and conduct a heat treatment under the conditions of 80° C. or higher. It should, however, be noted that in case where the heat treatment is conducted for too long a time period or at too high a temperature, the following problem newly arises. The nickel hydroxide serving as an active material, in X-ray diffractometry, comes to have reduced values of the peak intensity ratio between the (100) plane and (001) plane, (100)/(001), the peak intensity ratio between the (101) plane and (001) plane, (101)/(001), and the half width of the peak for the (101) plane, and the coefficient of use of the positive active material decreases accordingly (second problem).
“Background Art 2”
Since nickel/metal-hydride storage batteries, which are a kind of alkaline storage batteries, have a high energy density, they are extensively used as power sources for portable small electronic appliances including pocket telephones and small personal computers. The demand for the batteries is increasing remarkably.
Incidentally, a nickel/metal-hydride storage battery generally has a nickel electrode having a positive active material and a negative electrode having a hydrogen-absorbing alloy. This nickel electrode contains a low-order cobalt compound in which the cobalt has an oxidation number of 2 or smaller, e.g., cobalt hydroxide, so as to have enhanced conductivity and thereby improve the coefficient of use of the active material. During initial charge, this low-order cobalt compound is oxidized into a high-order cobalt compound in which the cobalt has an oxidation number larger than 2. The cobalt compound thus forms a conductive network to heighten the coefficient of use of the active material. This high-order cobalt compound is thought to be cobalt oxyhydroxide.
However, there have been cases where when alkaline storage batteries having such a nickel electrode are overdischarged, the conductive network is impaired and the restoration of discharge capacity becomes insufficient. The same applies in the case of long-term discharge at a high temperature.
Nickel/metal-hydride storage batteries further have the following drawback. At temperatures around ordinary temperature, the difference between the potential at which oxygen is evolved by the decomposition of the water constituting the electrolytic solution, i.e., oxygen evolution potential, and the potential at which the reaction in which the nickel hydroxide as the positive active material is oxidized to nickel oxyhydroxide occurs, i.e., oxidation reaction potential, is large and, hence, a high charge efficiency can generally be expected. At high temperatures, however, the difference between the oxygen evolution potential and the oxidation reaction potential becomes small and, hence, the charge efficiency tends to decease. Since nickel/metal-hydride storage batteries in many cases are usually disposed in small spaces, the temperature increase caused by, e.g., heat generation during charge/discharge is unavoidable and it is difficult to maintain a high charge efficiency in many cases.
Various techniques for inhibiting the charge efficiency of nickel/metal-hydride storage batteries from decreasing at high temperatures have been investigated. Examples thereof include (6) the technique described in JP-A-3-78965, JP-A-7-45281, etc., and (7) the technique described in JP-A-9-92279, JP-A-5-28992, JP-A-11-250908, etc.
However, the technique (6) shown above has not been always satisfactory in “the effect of inhibiting the charge efficiency from decreasing at high temperatures”. Furthermore, in the technique (7) shown above, it is difficult to obtain a sufficient charge efficiency from the beginning and there have been cases where a decrease in energy density occurs or no effect is observed (third problem).
“Background Art 3”
Recently, alkaline storage batteries such as nickel/metal-hydride storage batteries, nickel-cadmium storage batteries, and nickel-zinc storage batteries have come to be used as power sources for high-rate charge/discharge in power tools, hybrid electromobiles, and the like. The demand for these batteries is increasing rapidly.
Incidentally, nickel electrodes for use as the positive electrodes of alkaline storage batteries are classified into sinter electrodes and non-sinter electrodes. The sinter electrodes are obtained by depositing a positive active material comprising nickel hydroxide on a sintered substrate. The non-sinter electrodes are obtained by preparing a slurry of positive active material particles comprising nickel hydroxide using a thickener and other ingredients and coating or impregnating a foamed metallic substrate or the like with the slurry. Because the non-sinter electrodes can attain a higher capacity and because of the ease of production thereof, etc., they are coming to be frequently used in place of the sinter electrodes.
However, the non-sinter electrodes heretofore in use have had a problem that they are inferior in high-rate charge/discharge characteristics to the sinter electrodes. The main reasons for this are that the distance between the substrate and the positive active material particles is long and that the contacts between the positive active material particles differ in conductivity. An attempt was hence made to improve high-rate charge/discharge characteristics by adding a cobalt compound to a positive active material (see, for example, JP-A-62-256366). Namely, the cobalt compound added first dissolves in an alkaline electrolytic solution and thereafter reprecipitates as cobalt hydroxide, upon which reprecipitation the cobalt compound forms a network connecting the positive active material particles and the substrate. This network changes into a satisfactory conductive network when the cobalt hydroxide is oxidized into a highly conductive cobalt compound by the first charge of the battery. Consequently, high-rate charge/discharge characteristics are improved.
On the other hand, alkaline storage batteries have a drawback that when charge and discharge are repeated at a high current, the battery temperature increases and the battery which is still in an insufficiently cooled state shifts into the succeeding mode of charge or discharge. In particular, alkaline storage batteries are coming to be more frequently used in a high-temperature environment. In batteries of the type made up of nickel/metal-hydride storage cells assembled together, temperature unevenness among the individual cells has become large. Nickel/metal-hydride storage batteries have further had a problem that the charge efficiency thereof at high temperatures is considerably low. The reason for this is that during charge at high temperatures, the difference between the oxidation potential for the nickel hydroxide and the oxygen evolution potential in the final stage of charge is small and, hence, competitive reactions respectively for oxidation and for oxygen evolution occur. It has therefore been proposed to add to a nickel electrode a compound having the effect of shifting the oxygen evolution potential of a nickel electrode in the final stage of high-temperature charge to the nobler side. Specifically, this compound is a rare-earth compound. This technique is disclosed, for example, in
(8) JP-A-5-28992, JP-A-6-150925, JP-A-8-195198, JP-A-9-92279, etc.
However, there have been cases where the addition of a rare-earth compound arouses troubles that the rare-earth compound inhibits the dissolution of cobalt ions in the electrolytic solution and the precipitation of cobalt hydroxide from the electrolytic solution or the rare-earth compound dissolves in the electrolytic solution and deposits on the negative electrode to heighten the resistance of the negative electrode. There has hence been a problem in the addition of a rare-earth compound that high-rate discharge characteristics deteriorate although the high-temperature charge efficiency is improved.
On the other hand, the above-described technique in which cobalt hydroxide is oxidized by the first charge of the battery to thereby form a satisfactory conductive network has a drawback that since the oxidation of the cobalt hydroxide by charge is an irreversible reaction, the negative electrode at the time of discharge termination has electricity remaining undischarged. A discharge reserve is generated also by the corrosion of the hydrogen-absorbing alloy employed in the negative electrode. Furthermore, for inhibiting a sealed alkaline storage battery from undergoing hydrogen evolution at the negative electrode in the final stage of charge, it is necessary to impart a charge reserve to the negative electrode. This impartation of a charge reserve inhibits hydrogen evolution at the negative electrode and accelerates the absorption of the oxygen evolved at the positive electrode, so that the internal pressure of the battery can be inhibited from increasing in the final stage of charge. Consequently, increasing a charge reserve by reducing a discharge reserve makes it possible to inhibit the internal pressure of the battery from increasing during high-rate charge. Furthermore, since a discharge reserve gradually accumulates with charge/discharge cycles to reduce the charge reserve, a reduction in discharge reserve improves charge/discharge cycle characteristics. Moreover, the increase in charge reserve by a reduction in discharge reserve makes it possible to inhibit the hydrogen-absorbing alloy in the negative electrode from becoming finer particles. The hydrogen-absorbing alloy thus inhibited from becoming finer particles is inhibited from corrosion and makes it possible to inhibit the generation of a discharge reserve.
Incidentally, the following has been proposed as a technique for attaining a reduction in discharge reserve. Namely, as explained above in (5) under “Background Art 1”, an attempt has been made in JP-A-2000-223119 in which a high-order cobalt compound having a cobalt oxidation number larger than 2 is deposited on the surface of positive active material particles comprising nickel hydroxide and the nickel hydroxide is partly oxidized with an oxidizing agent in an aqueous alkali solution.
However, the positive active material obtained by the synthesis method described above has had the following problems. Namely, as pointed out as the “first problem”, when the oxidation treatment is conducted in a high-concentration aqueous alkali solution at a high temperature, part of the nickel hydroxide is oxidized to γ-NiOOH and the tap density of the positive active material particles decreases. There is hence a problem that the decrease in tap density is contrary to the desired density increase in positive active materials and this in turn is contrary to the desired capacity increase. On the other hand, when the oxidation treatment is conducted in a low-concentration aqueous alkali solution at a low temperature, not only the coefficient of use of the positive active material in discharge decreases mainly due to an increased powder resistance, but also an inactive nickel oxide is yielded as a by-product. Thus, in the oxidation treatment in an aqueous alkali solution, the coefficient of use of the positive active material in discharge is inconsistent with the tap density of the active material (fourth problem).